1. Field of the Invention
The present invention relates to a ladder-type filter comprising at least one series resonator and at least one parallel resonator which are connected with each other in the form of a ladder, and more particularly, the present invention relates to a ladder-type filter which has an improved resonator structure forming the series and parallel resonators.
2. Description of the Background Art
FIG. 1 shows an exemplary structure of a conventional ladder-type filter. This ladder-type filter is formed by a plurality of angular piezo-resonators utilizing an expansion vibration mode. Namely, a two-stage ladder-type filter having four elements shown in a circuit diagram in FIG. 2 is formed by rectangular plate type series resonators 1 and 2 and rectangular plate type parallel resonators 3 and 4.
Referring to FIG. 1, numeral 2a denotes an electrode which is formed on one major surface of the series resonator 2, while a similar electrode is also formed on another major surface of the series resonator 2. Further, similar electrodes are also formed on both major surfaces of the series resonator 1. On the other hand, electrodes 3a and 4a are formed entirely over both major surfaces of the series resonators 3 and 4, respectively.
Numerals 5 to 11 denote metal terminals, which are employed to electrically connect the series resonators 1 and 2 and the parallel resonators 3 and 4 with each other as shown in FIG. 2. These metal terminals 5 to 11 are stored in a case member 12, which is made of an insulating material, with the series resonators 1 and 2 and the parallel resonators 3 and 4. A cover member (not shown) closes an upper opening 12a of the case member 12, thereby forming a ladder-type filter component. The metal terminals 9 to 11 are drawn out from the case member 12, to be employed as terminals for connection with an exterior component.
In order to drive the aforementioned ladder-type filter, it is necessary that the series resonators 1 and 2 and the parallel resonators 3 and 4 which are stored in the case member 12 can vibrate in desired modes. In other words, the resonators 1 to 4 must not be inhibited from vibration in the case member 12. Therefore, the metal terminal 11 which is located on an end portion is formed of a spring terminal having a spring property.
In the ladder-type filter shown in FIG. 1, however, a considerably large unnecessary space is defined due to the metal terminal 11 which is formed by a spring terminal for allowing vibration of the resonators 1 to 4 stored in the case member 12, and hence, the overall ladder-type filter has a considerably large size. For example, the completed two-stage ladder-type filter containing four elements shown in FIG. 1 is about 7.0 mm by 8.0 mm by 8.0 mm.
In recent years, there has been a demand for a ladder-type filter which is formed as a surface-mount type electronic component.
To this end, co-pending U.S. patent application Ser. No. 07/941,081 and International Patent Laying-Open No. WO 92/16997 describe a ladder-type filter which can be reduced in overall size and formed as a surface-mount type electronic component. In this ladder-type filter, series and parallel resonators are formed by tuning fork type piezo-resonators defining tuning fork type vibrating parts on one edge of a piezoelectric plate. Further, a plurality of tuning fork type piezo-resonators forming the series and parallel resonators are stacked with each other through a cavity forming member for ensuring cavities for allowing vibration of the tuning fork type vibrating parts, to be integrated with each other.
In the aforementioned ladder-type filter employing tuning fork type piezo-resonators, it is possible to implement simplification of assembling steps, miniaturization and surface mounting. In this ladder-type filter, however, it is impossible to ensure a sufficient bandwidth, due to employment of the tuning fork type piezo-resonators.